(1)Study on the QoS control at TCP/IP routerOne of the purposes of this research is to propose a new algorithm of packet control at a router which solves problems of V-WFQ. In V-WFQ, the fairness of throughputs degrades if TCP and UDP connections are aggregated at a router, and the queue length becomes long if some connections join or leave at a router. To solve these problems, we propose an algorithm which uses continuous counter and deleting counter to count the present number of active connections. We confirmed the performance of our method compared to other conventional methods.(2)Study on a fast connection admission control which guarantees the packet loss probabilityWe investigate the exponential decay of the tail probability of the queue length in the steady state and give a sufficient condition for the exponential decay. A result obtained in this research is a theorem that if the real point of the axis of convergence of the Laplace-Stieltjes transform of the distribution function of the queue length is a pole, then the tail probability decays exponentially. This result is so general that it can be applied to many type of queues. The result of this research was submitted to the IEEE transactions.(3)Evaluation of delay time in a packet networkWe study a deterministic upper bound for the end-to-end delay time of a packet. It is assumed that a traffic stream is shaped by a token bucket algorithm. In this study, we proposed a new upper bound which is more accurate than other conventional bounds by noticing that the aggregated burst is processed as time goes. We confirmed that our upper bound is superior to other bounds by computer simulation.